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. Author manuscript; available in PMC: 2011 Feb 1.
Published in final edited form as: J Educ Psychol. 2010 Feb 1;102(1):32–42. doi: 10.1037/a0017288

Table 2.

Intraclass Correlations for Same- and Different-Class Twins Pairs (N Pairs in Parentheses; Contrasts Significant at .01 in Bold)

Grade Test MZ pairs DZ pairs

US Australia Total US Australia Total

S D S D S D S D S D S D
Kindergarten (171, 53) (85, 46) (256, 99) (174, 85) (66, 31) (240, 116)
TOWRE SWE .89 .84 .85 .82 .88 .83 .64 .51 .51 .41 .61 .50
TOWRE PDE .76 .61 .73 .54 .75 .59 .55 .53 .38 .34 .51 .52
Spelling .78 .75 .83 .73 .80 .75 .59 .52 .46 .55 .56 .55
Total literacy .88 .79 .90 .76 .89 .78 .59 .57 .54 .42 .58 .54
Grade 1 (95, 124) (58, 66) (153, 190) (97, 156) (45, 46) (142, 202)
TOWRE SWE .88 .86 .84 .76 .86 .83 .49 .48 .36 .32 .46 .45
TOWRE PDE .83 .73 .83 .80 .81 .76 .34 .33 .44 .44 .41 .41
WRAT Spelling .82 .73 .84 .60 .82 .69 .48 .41 .23 .33 .41 .39
WPC .74 .82 .82 .66 .77 .77 .58 .40 .51 .29 .56 .37
Total literacy .88 .86 .90 .74 .89 .82 .55 .46 .36 .34 .50 .44
Grade 2 (73, 105) (43, 68) (116, 173) (55, 158) (34, 40) (89, 198)
TOWRE
SWE
.88 .79 .79 .78 .84 .79 .55 .41 .46 .24 .52 .38
TOWRE
PDE
.85 .75 .80 .83 .83 .78 .50 .46 .35 .20 .45 .42
WRAT
Spelling
.84 .77 .78 .77 .81 .76 .54 .35 .35 −.05 .48 .29
WPC .72 .70 .76 .66 .73 .68 .56 .38 .52 .16 .54 .33
Total
literacy
.90 .84 .84 .76 .88 .81 .60 .47 .40 .24 .55 .41

Note: S = Same class, D = Different class; TOWRE SWE = Test of Word Recognition Efficiency, Sight Word Efficiency; TOWRE PDE = Test of Word Recognition Efficiency, Phonemic Decoding Efficiency; WRAT Spelling = Wide Range Achievement Test, Spelling; WPC = Woodcock Reading Mastery Test (Revised), Passage Comprehension. Total literacy in each grade is based on mean of all tests at that grade for each child.